In a move to comfort potential switchers who would leave AT&T for Verizon Wireless as the iPhone 4 is scheduled to arrive early next month to AT&T’s rival CDMA carrier, AT&T is bringing back its unlimited data plans, which the carrier had stopped offering to new customers, to retain its subscribers and hope to avert losses.

While AT&T had allowed users who had signed up for the $30 unlimited data plan to continue to keep their plans, it had cautioned those users grandfathered into the unlimited plans that if they switch to a finite, capped data plan for a cheaper monthly rate, they would lose their option for the unlimited plan in the future. New users who signed up after AT&T had stopped offering the $30 unlimited package must sign up for a capped plan–the maximum capped plan is $25 per month for 2 GB of data.

However, the carrier is starting to let some users who were previously on an unlimited plan and switched to a finite plan to switch back when they call in. The catch is that once upon a time, you must have had been on an unlimited data plan. Although AT&T’s spokespeople wouldn’t comment on the matter with USA Today, the move essentially helps to alleviate users who may be tempted to switch to rival Verizon Wireless for an unlimited data plan.

The allure of a wireless all-you-can-eat data plan makes sense considering all the data-centric features that smartphones now come with, including music download, YouTube video viewing, photo and video uploads, Netflix and Hulu streaming, and other apps that can potentially consume massive amounts of data on top of just push email.

As AT&T plays its network strengths over rival Verizon Wireless, hopefully, it may begin to open up unlimited smartphone data to new users or those without an unlimited data plan previously. The carrier has been boasting qualities such as faster 3G download speeds on its HSPA network compared to Verizon’s EV-DO data speeds as well as simultaneous voice and data capabilities. These features, along with the ease in switching phones with a SIM card, may still prompt new users to choose AT&T. Having an unlimited data option will certainly make it easier for those users who are new to the carrier to pick a GSM iPhone on AT&T.